This report reviews the Internal Revenue Service's (IRS) third expenditure plan for its systems modernization project. GAO found that the plan satisfied the conditions specified in Treasury's 1998 and 1999 appropriations acts and that IRS was making progress towards satisfying Congress' direction on the Custodial Accounting Project (CAP) and Security and Technology Infrastructure Release (STIR) Project. Although IRS has made significant progress in establishing effective modernization management capability, important and challenging work remains to ensure that the systems work as intended. GAO also found that five modernization initiatives experienced schedule delays or cost increases, each of which IRS disclosed in the third plan. However, the third plan did not address whether projects' prior commitments for delivery of promised systems capabilities and benefit/business value were being met. IRS used contractor-provided "rough order-of-magnitude" estimates in preparing its third expenditure plan. However, consistent with its established practice, IRS planned to validate the third plan's estimates as part of its negotiating and definitizing contract task orders. For IRS' second expenditure plan, this process resulted in finalized contract costs that were $9 million under the "rough order-of-magnitude" estimates in the plan.